Shock wave reflector and detonation chamber

ABSTRACT

A shock wave reflector includes a number of reflective units positioned along a longitudinal direction and separated by a gap G. Each reflective unit has a length L. The length L and the gap G are governed by a relationship L+G≧λ. The variable λ characterizes a cell size for a detonation mixture. A detonation chamber includes a receiving end, a discharge end, and a wall extending along a longitudinal direction between the receiving and discharge ends. The detonation chamber further includes a number of reflective units formed in the wall and positioned along the longitudinal direction. The reflective units are separated by a gap G, and each reflective unit has a length L.

[0001] This invention was made with Government support under contractnumber DABT 63-00-C-001 awarded by DARPA. The Government has certainrights in the invention.

BACKGROUND

[0002] The invention relates generally to pulse detonation engines and,more particularly, to enhancement of detonation for pulse detonationengines.

[0003] Pulse detonation engines detonate a fuel and oxidizer mixture,producing hot combustion gases, which have increased temperature andpressure and are propagated at supersonic speeds. The hot combustiongases are directed from the engine to produce thrust.

[0004] A representative configuration for detonation for a pulsedetonation engine is illustrated in FIG. 1. As shown, a spark initiatesthe detonation process. If the spark has enough energy for the fuel andair mixture, a shock is initiated and travels to the right. As the shockprocesses the fuel and air mixture and turbulence is developed,formation of a transverse wave structure is initiated. Reflection of thetransverse shock waves from the walls of the detonation chamber (shownhere as cylindrical) creates interactions between the transverse shockwaves, which result in “hot spots,” which have high local values oftemperature and pressure and seed detonation.

[0005] Exemplary fuel and air mixtures for pulse detonation enginesinclude liquid fuel and air mixtures. One problem with liquid fuel/airdetonation is a long deflagration-to-detonation transition (DDT) length,which is typically larger than several meters.

[0006] Attempts have been made to decrease the DDT length by placingobstacles inside a detonation chamber, such as the augmentation devicediscussed in U.S. Pat. No. 5,901,550, by Bussing et al. and assigned toAdroit Systems, Inc. The augmentation device consisted of threading theinterior surface of the inlet end of the detonation chamber with ahelical-type thread to provide a ridged surface. Other attempts todecrease the DDT length include using pre-detonators and improving thecombination of spark energy and position, detonation chamber geometry,and fuel/air properties.

[0007] Although some success has been achieved, shorter DDT lengthsremain a central challenge for liquid fuel detonation systems. It wouldtherefore be desirable to reduce the DDT length and, more particularly,to provide a detonation chamber having a reduced DDT length.

BRIEF DESCRIPTION

[0008] Briefly, in accordance with one embodiment of the presentinvention, a shock wave reflector is disclosed and includes a number ofreflective units positioned along a longitudinal direction and separatedby a gap G. Each reflective unit has a length L. The length L and thegap G are governed by a relationship L+G≧λ. The variable λ characterizesa cell size for a detonation mixture.

[0009] In accordance with another embodiment of the present invention, adetonation chamber is disclosed and includes a receiving end, adischarge end, and a wall extending along a longitudinal directionbetween the receiving and discharge ends. The detonation chamber furtherincludes a number of reflective units formed in the wall and positionedalong the longitudinal direction. The reflective units are separated bya gap G, and each reflective unit has a length L.

DRAWINGS

[0010] These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

[0011]FIG. 1 illustrates a typical configuration for detonation for apulse detonation engine;

[0012]FIG. 2 illustrates a cellular pattern for detonation;

[0013]FIG. 3 illustrates a detonation chamber embodiment and a shockwave reflector embodiment of the invention in cross-sectional view;

[0014]FIG. 4 shows the detonation chamber of FIG. 3 in perspective view;

[0015]FIG. 5 shows a shock wave reflector embodiment having reflectiveunits, which are semi-elliptical in cross section;

[0016]FIG. 6 shows a shock wave reflector embodiment having reflectiveunits, which are open polygons in cross section;

[0017]FIG. 7 illustrates alternative shock wave reflector embodiment ofthe invention, which incorporates cavities;

[0018]FIG. 8 is a cross-sectional view of a reflective unit of the shockwave reflector of FIG. 7;

[0019]FIG. 9 illustrates an alternative detonation chamber embodimentincluding a slidably configured liner, which is in an open position;

[0020]FIG. 10 shows the detonation chamber of FIG. 9 with the liner in ashielding position;

[0021]FIG. 11 illustrates another detonation chamber embodiment and aspiral reflective unit embodiment of the invention in cross-sectionalview; and

[0022]FIG. 12 shows the detonation chamber of FIG. 10 in perspectiveview.

DETAILED DESCRIPTION

[0023] A shock wave reflector 10 embodiment of the present invention isdescribed with reference to FIGS. 3 and 4. As seen in FIG. 3 incross-sectional view, the shock wave reflector 10 includes a number ofreflective units 1 positioned along a longitudinal direction 2.Neighboring reflective units 1 are separated by a gap G, and eachreflective unit has a length L, as indicated in FIG. 3. The length L andthe gap G are governed by a relationship:

L+G≧λ.

[0024] As used here, the variable λ characterizes the cell size for adetonation mixture.

[0025] By way of background, cell size λ is a fundamental property ofdetonations. More particularly, cell size λ is a function of the initialtemperature T₀ and pressure P₀ and of the detonation mixture, namely ofthe fuel and oxidizers composing the detonation mixture. A schematicdiagram of a cellular structure 52 associated with detonations isillustrated in FIG. 2. A cellular pattern 50 results from interactionsbetween transverse shock waves 60 traveling in a latitudinal direction 3behind the detonation 52. The intersection points (or “hot spots”) 62 oftransverse shock waves 60 have high local temperature T and pressure Pvalues, and detonation is seeded at intersection points 62.

[0026] Referring back to FIGS. 3 and 4, exemplary reflective units 1 areannular. Although reflective units 1 are shown in FIG. 3 as beingsemi-circular in cross-section, other exemplary reflective units 1 aresemi-elliptical or open-polygonal in cross-section, as shown for examplein FIGS. 5 and 6, respectively. By the terms “semi-elliptical” and“open-polygonal,” it is meant that the cross-sections correspond to aportion of an ellipse or to an open polygon, respectively. Further,although reflective units 1 are shown as being smooth, reflective unitsmay also be jagged.

[0027] Beneficially, shock wave reflector 10 reduces thedeflagration-to-detonation transition length (DDT), thereby enhancingdetonation. By reflecting transverse shock waves 60 from reflectiveunits 1, energy is focused at “hot spots” 62, producing high localtemperature T and pressure P values at hot spots 62. In this manner, thetransition to detonation is enhanced, by producing the hot spots over ashorter longitudinal distance. More particularly, by setting the sum ofthe length L and gap G equal to be equal to or to exceed the cell size λfor the detonation mixture, full transverse shock waves are enclosed bythe pattern formed by the reflective units 1, focusing the energy storedin the transverse shock waves to create hot spots 62 over a shorterdistance.

[0028] To further enhance the focusing of energy at hot spots 62,according to a particular embodiment length L and gap G are governed bya relationship:

L+G=nλ.

[0029] As used here, the variable n denotes an integer.

[0030] According to another particular embodiment, shock wave reflector10 includes at least ten (10) reflective units 1. Beneficially, thisconfiguration further enhances hot spot seeding due to the number ofreflective units.

[0031] An alternative shock wave reflector 10 embodiment is illustratedin FIG. 7. For this embodiment, each reflective unit 1 includes a cavity3. Exemplary cavities 3 are elliptical, as shown in FIG. 7. Otherexemplary cavities 3 are hemispheres or open polyhedrons (not shown).According to a particular embodiment, shock wave reflector 10 includesat least ten (10) cavities 3.

[0032] For reflection enhancement, in a more particular embodiment, eachreflective unit 1 (indicated by dashed line in FIG. 7) includes a numberof cavities 3 positioned at a respective number of angular orientations.Reflective unit 1 is shown in cross-sectional view in FIG. 8. Fordetonation enhancement, shock wave reflector 10 according to a moreparticular embodiment includes at least ten (10) reflective units 1. Forthe shock wave reflector 10 embodiment shown in FIG. 7, the length L ofreflective units 1 and gap G between neighboring reflective units 1 aregoverned by the relationship L+G≧λ, only in a preferred embodiment.

[0033] A detonation chamber 20 embodiment is described with reference toFIGS. 3 and 4. As seen in FIG. 3 in cross-sectional view, detonationchamber 20 includes a receiving end 22, a discharge end 24, and a wall26 extending along a longitudinal direction between receiving anddischarge ends 22, 24. Fuel and oxygen are introduced at receiving end22. Exemplary fuel types include hydrogen, propane, JP10, JP8, JetA,C₂H₂, and C₂H₄. Exemplary oxygen sources include O₂ and air, for exampleliquid O₂ and liquid air. However, the invention is not limited to anyparticular fuel/oxygen mixtures. As seen in FIG. 3, detonation chamberfurther includes a number of reflective units 1 formed in wall 26 andpositioned along longitudinal direction 2. Reflective units 1 areseparated by gap G and have length L. According to a particularembodiment, length L and gap G are governed by the relationship L+G≧λ,to enhance the focusing of transverse shock waves by reflective units 1.One exemplary material for wall 26 and reflective units 1 is stainlesssteel. However, the invention is not limited to any specific materials.

[0034] According to a particular embodiment, reflective units 1 areintegral to wall 26, meaning that the reflective units and wall 26 areeither machined from a single piece (not shown) or are attached in acontinuous manner, for example by welding.

[0035] According to a particular embodiment, at least one of reflectiveunits 1 is formed in a vicinity of receiving end 22. By the phrase “inthe vicinity,” it is meant that the reflective unit in question iscloser to receiving end 22 than to discharge end 24.

[0036] In one embodiment, each reflective unit 1 extends around an innersurface 28 of wall 26. As discussed above, exemplary reflective units 1are semi-circular, semi-elliptical, or open-polygons in cross-section.According to a more particular embodiment, reflective units 1 areannular, as shown in FIGS. 3 and 4.

[0037] To further enhance detonation, detonation chamber 20 according toa more particular embodiment includes at least ten (10) reflective units1.

[0038] An alternative detonation chamber 20 embodiment is illustrated inFIG. 7 in side view. For this embodiment, each reflective unit 1includes a cavity 3. As discussed above, exemplary cavities 3 arehemispheric, elliptical, or open polyhedrons. To enhance detonation,detonation chamber 20 includes at least ten (10) cavities 3, accordingto a more particular embodiment.

[0039] For reflection enhancement, in a more particular embodiment eachreflective unit 1 (indicated by dashed line in FIG. 7) includes a numberof cavities 3 formed in wall 26 and positioned at a respective number ofangular orientations on inner surface 28 of wall 26. Reflective unit 1is shown in cross-sectional view in FIG. 8. For detonation enhancement,detonation chamber 20 according to a more particular embodiment includesat least ten (10) reflective units 1. For the detonation chamber 20embodiment shown in FIG. 7, the length L of reflective units 1 and gap Gbetween neighboring reflective units 1 are governed by the relationshipL+G≧λ, only in a preferred embodiment.

[0040] As known to those skilled in the art, the configuration of adetonation chamber 20 varies, depending on the use to which it is put.Exemplary uses include rockets, air breathing engines such as turbofanengines, and stationary power generators. However, for a particularembodiment, receiving end 22, discharge end 24, and wall 26 form adetonation tube (also indicated by reference numeral 20). The term“tube” is used here to mean a generally cylindrical shape, as shown forexample in FIGS. 3, 4, and 7. However, the present invention is notlimited to detonation tubes but encompasses detonation chambers havingother shapes that incorporate the features of the invention. Accordingto a more particular embodiment shown in FIGS. 9 and 10, detonationchamber 20 further includes a liner 40 positioned between receiving anddischarge ends 22, 24 within wall 26. Liner 40 is configured to slidebetween an open position, which is illustrated in FIG. 9 and a shieldingposition, which is illustrated in FIG. 10. In the open position, accessto reflective units 1 exists, whereas in the shielding position, liner40 blocks access to reflective units 1. Although shown only incross-section in FIGS. 9 and 10, liner 40 has the shape of a hollow tubewith a smooth inner surface 42, relative to the curved surface ofreflective units 1. The latter configuration with liner 40 isparticularly useful for detonation chambers 20 which are alternatelyused in detonation/deflagration and exhaust modes. For example, anafterburner is employed in military applications to alternate between anexhaust mode, for which a smooth surface (liner 40 in shieldingposition) is desirable, and a detonation/deflagration mode (liner 40 inopen position) to provide additional thrust, for which the reflectiveunits 1 are desirable.

[0041] Another detonation chamber 20 embodiment is illustrated in FIGS.11 and 12. As seen in FIG. 11, detonation chamber 20 for this embodimentincludes receiving and discharge ends 22, 24, wall 26, and a spiralreflective unit 30 formed in wall 26. Spiral reflective unit 30 extendsalong longitudinal direction 2 and includes a number of windings 32.Similar to the reflective units 1 discussed above, each winding haslength L, and neighboring windings are separated by gap G. Length L andgap G are governed by the relationship:

L+G≧λ,

[0042] which is discussed above with respect to reflective units 1.According to a more particular embodiment, the sum of length L and gap Gis equal to an integer multiple of the variable λ, to further enhancedetonation. Exemplary windings 32 are semi-circular, semi-elliptical, oropen-polygons in cross-section. Beneficially, spiral reflective unit 30increases ease of manufacturability for detonation chamber 20 and can beformed, for example, using a tap.

[0043] To enhance detonation, spiral reflective unit 30 according to aparticular embodiment includes an end 34 formed in a vicinity ofreceiving end 22. According to a more particular embodiment, dischargeend 24, and wall 26 form a detonation tube 20. For another embodiment,detonation chamber 20 further includes liner 40, as indicated in FIG. 11and discussed above.

[0044] While only certain features of the invention have beenillustrated and described herein, many modifications and changes willoccur to those skilled in the art. It is, therefore, to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit of the invention.

1. A shock wave reflector comprising: a plurality of reflective unitspositioned along a longitudinal direction and separated by a gap G, eachreflective unit having a length L, wherein the length L and the gap Gare governed by a relationship: L+G≧λ, wherein the variable λcharacterizes a cell size for a detonation mixture.
 2. The shock wavereflector of claim 1, wherein said reflective units are annular.
 3. Theshock wave reflector of claim 2, wherein said reflective units aresemi-circular in cross-section.
 4. The shock wave reflector of claim 2,wherein said reflective units are semi-elliptical or open-polygons incross-section.
 5. The shock wave reflector of claim 2, wherein saidplurality of reflective units comprises at least ten reflective units.6. The shock wave reflector of claim 1, wherein each of said reflectiveunits comprises a cavity.
 7. The shock wave reflector of claim 6,wherein said cavity is hemispheric.
 8. The shock wave reflector of claim6, wherein said cavity is semi-elliptical or an open polyhedron.
 9. Theshock wave reflector of claim 6, wherein said plurality of reflectiveunits comprises at least ten cavities.
 10. The shock wave reflector ofclaim 1, wherein each of said reflective units comprises a plurality ofcavities positioned at a respective plurality of angular orientations.11. The shock wave reflector of claim 10, wherein each cavity ishemispheric.
 12. The shock wave reflector of claim 10, wherein eachcavity is semi-elliptical or an open polyhedron.
 13. The shock wavereflector of claim 10, wherein said plurality of reflective unitscomprises at least ten reflective units.
 14. The shock wave reflector ofclaim 1, wherein the length L and the gap G are governed by arelationship: L+G=nλ, the variable n denoting an integer.
 15. Adetonation chamber comprising: a receiving end; a discharge end; a wallextending along a longitudinal direction between said receiving anddischarge ends; and a plurality of reflective units formed in said wall,positioned along the longitudinal direction, and separated by a gap G,each reflective unit having a length L.
 16. The detonation chamber ofclaim 15, wherein the length L and the gap G are governed by arelationship: L+G≧λ, wherein the variable λ characterizes a cell sizefor a detonation mixture.
 17. The detonation chamber of claim 16,wherein at least one of said reflective units is formed in a vicinity ofsaid receiving end.
 18. The detonation chamber of claim 17, wherein eachof said reflective units extends around an inner surface of said wall.19. The detonation chamber of claim 18, wherein said reflective unitsare semi-circular, semi-elliptical, or an open polygon in cross-section.20. The detonation chamber of claim 19, wherein said reflective unitsare annular.
 21. The detonation chamber of claim 20, wherein saidplurality of reflective units comprises at least ten reflective units.22. The detonation chamber of claim 17, wherein each of said reflectiveunits comprises a cavity.
 23. The detonation chamber of claim 22,wherein said cavity is hemispheric, semi-elliptical, or an openpolyhedron.
 24. The detonation chamber of claim 23, wherein saidplurality of reflective units comprises at least ten cavities.
 25. Thedetonation chamber of claim 15, wherein each of said reflective unitscomprises a plurality of cavities formed in said wall and positioned ata respective plurality of angular orientations on said inner surface ofsaid wall.
 26. The detonation chamber of claim 25, wherein the length Land the gap G are governed by a relationship: L+G≧λ, wherein thevariable λ characterizes a cell size for a detonation mixture.
 27. Thedetonation chamber of claim 26, wherein each cavity is hemispheric,semi-elliptical, or an open polyhedron.
 28. The detonation chamber ofclaim 27, wherein said plurality of reflective units comprises at leastten reflective units.
 29. The detonation chamber of claim 15, whereinsaid receiving end, discharge end, and wall form a detonation tube. 30.The detonation chamber of claim 25, wherein the length L and the gap Gare governed by a relationship: L+G≧λ, wherein the variable λcharacterizes a cell size for a detonation mixture.
 31. The detonationchamber of claim 30, further comprising: a liner positioned between saidreceiving and discharge ends within said wall, wherein said liner isconfigured to slide between an open position and a shielding position.32. The detonation chamber of claim 16, wherein the length L, and thegap G are governed by a relationship: L+G=nλ, the variable n denoting aninteger.
 33. A detonation chamber comprising: receiving end; a dischargeend; a wall extending along a longitudinal direction between saidreceiving and discharge ends; and spiral reflective unit formed in saidwall, extending along the longitudinal direction, and comprising aplurality of windings, each winding having a length L, and said windingsbeing separated by a gap G, wherein the length L and the gap G aregoverned by a relationship: L+G≧λ, wherein the variable λ characterizesa cell size for a detonation mixture.
 34. The detonation chamber ofclaim 33, wherein said spiral reflective unit includes an end formed ina vicinity of said receiving end.
 35. The detonation chamber of claim34, wherein said windings are semicircular, semi-elliptical, or an openpolygon in cross-section.
 36. The detonation chamber of claim 35,wherein said receiving end, discharge end, and wall form a detonationtube.
 37. The detonation chamber of claim 36, further comprising: aliner positioned between said receiving and discharge ends within saidwall, wherein said liner is configured to slide between an open positionand a shielding position.
 38. The detonation chamber of claim 33,wherein the length L and the gap G are governed by a relationship:L+G=nλ, the variable n denoting an integer.
 39. A spiral reflective unitextending along a longitudinal direction, said spiral reflective unitcomprising: a plurality of windings, each winding having a length L, andsaid windings being separated by a gap G, wherein the length L and thegap G are governed by a relationship: L+G≧λ, wherein the variable λcharacterizes a cell size for a detonation mixture.
 40. The spiralreflective unit of claim 39, wherein said windings are semicircular,semi-elliptical, or an open polygon in cross-section.
 41. The spiralreflective unit of claim 40, wherein the length L and the gap G aregoverned by a relationship: L+G=nλ, the variable n denoting an integer.42. A shock wave reflector comprising a plurality of reflective unitspositioned along a longitudinal direction, wherein each reflective unitcomprises a plurality of cavities positioned at a respective pluralityof angular orientations.
 43. The shock wave reflector of claim 42,wherein each cavity is hemispheric, semi-elliptical or an openpolyhedron.
 44. The shock wave reflector of claim 43, wherein saidplurality of reflective units comprises at least ten reflective units.